During operation of an intermittently discharging centrifugal separator, sludge collected in the radially outer portion of the separation space needs to be discharged in order to maintain a good separation efficiency. On the other hand, discharge is a disturbance in the process of separation and thus it may be sought to have a low frequency of discharge. By indicating the level of sludge in the separation space and to initiate discharge when the sludge reaches a certain level the discharge timing can be optimised.
Several techniques for indicating the level of sludge in a centrifugal separator are previously known wherein the level of sludge in the separation space is indicated by means of indicating channels extending from the separating space towards centre of the rotor. The indicating channels are adapted for passage of an indicating liquid through its radially outer end which is positioned to be blocked by sludge collected to a predetermined level in the separating space.
GB 1099256 A describes a centrifugal separator rotor provided with two indicating channels extending from two different radial distances from the rotor axis, whereby the level of sludge may be indicated by monitoring the difference in pressure, flow or turbidity in the two indicating channels.
U.S. Pat. No. 3,642,196 A describes another a centrifugal rotor with one indicating channel and an arrangement which provides a measurement of the difference between the pressure in the indicating channel and the pressure of the separated liquid.
A drawback with these solutions is that the rotor has to be provided with an extra feature, i.e. at least one indicating channel, whereby retrofitting is more difficult to obtain.